By agreeing to buy 30 single-aisle jetliners from Boeing Co., Japan Airlines has turned to the Chicago company for a second big order in almost as many months. The decision stymied rival supplier Airbus's effort to expand its presence in one of Asia's biggest aircraft markets.

The deal, valued at $1.8 billion based on the list price for Boeing's new 737NG airplane, solidifies the U.S. aerospace company's dominance in Japan at a time...